Electric foil resistance drier



P. EISLER ELECTRIC FOIL RESISTANCE PRIER July 30, 1963 2 Sheets-Sheet 1 Filed Dec. 29, 1958 ffgz v July 30, 1963 P. ElsLER ELECTRIC Fon. RESISTANCE PEIER 2 Sheets-Sheet 2 Filed Deo. 29, 1958 /lvl/EA/TOAJ Pau/ Els/er 3,099,540 ELECTRIC FOIL RESISTANCE DRIER Paul Eisler, '7 Exeter Road, London NW. 2, England Filed Dec. 29, 1953, Ser. No. '783,609 Claims priority, application Great Britain Jan. 7, 1958 9 Claims. (Cl. 34-1) The present invention relates to drying of Wet structures, including parts of the human body as well as durable consumer goods, and comprises the evaporation of absorbed vapours or liquids and the setting or curing of paint and synthetic resin compositions. In particular it relates to drying by the aid of heat, of relatively thin layers in which it is of importance that the tempera-ture should not exceed a safe limit at any point. An object of the invention is to accomplish such heating electrically at a high rate, in an economic and convenient manner. Other .objects will become apparent as the description p-roceeds.

The invention provides a method of drying a durable structure :at rest which includes a layer having absorbed therein a fluid and requiring to be heated to dry the layer, which comprises hxing a heating film to said structure in heat conductive connection with said layer over an area corresponding to substantially the whole area of said layer, a major part of the area of the film being occupied by an electrically conductive surface pattern substantially uniformly distributed over it, and passing through the surface pattern an electric current of sufficient intensity to raise its temperature but only to a value below that at which the combination of the pattern and the structure .would be damaged during the time of application of the heat.

It is to be understood that the term durable structure at rest means that the structure does not consist of consumable goods or loose bulk material such as grain or chemicals and tha-t movement as such has no part in the drying process. The drying of such goods and materials is a different problem from that with which the present invention is concerned.

The invention also provides a combination comprising a durable structure which includes at least a layer having absorbed therein a fluid and requiring to be heated to dry the layer, and a heating lrn fixed to said structure so that the lm and structure are relatively at rest, said film .having a heat conductive connection wi-th said layer over an area corresponding to substantially the whole area of said layer, a .major part of the area of the ilm being occupied by an electrically conductive surface pattern having at least two terminal zones and being substantially uniformly distributed over the area with substantially constant resistance per unit area and aggregating in a small total resistance of the whole pattern between ter-minal zones so that the voltage drop produced by a relatively heavy current across the said terminal zones is of such value that the watt loss in the heating film including the terminal zones is at no point capable of raising its temperature to a value at which the combination of the tilm and the structure would be damaged during the time of connection to the supply.

It will be clear that the heating lm being a very thin material, is of small mass and therefore of very low heat inertia. It Will also be clear that such a thin material is very liexible.

By the expression relatively heavy current is meant a current in the range of amperes, as distinct from milliamperes or kilo-amperes, and by the expression small total resistance is meant a resistance in the range of ohms as distinct from milli-ohms or kilo-ohms. Desirably the voltage drop is of such low value that it is not dangerous to human contact, that is usually below 50 volts.

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It will be clear that the development of heat must not be greater per unit area of the terminal zones than elsewhere and this is preferably achieved by making the cross- `sectional area of each terminal zone not less than the aggregate cross-sectional area of the pattern corresponding to the Itotal resistance. In general there will usually be a number of paths in parallel between the two terminal Zones and the cross-sectional area of each terminal zone will then be not less than the total cross-sectional areas of the several parallel paths. An alternative way of keeping down the development of heat at the terminal zones would be to add metal of higher conductivity to them as by clamping or plating.

Since the area occupied by the lilm substantially corresponds to the area of the surface to be treated and since the major part of the area is occupied by the conductive pattern substantially uniformly distributed over it, maximum use of the area -available is made while anything in the nature of hot spots is avoided so that a maximum rate of heat dissipation for the particular vconditions can be achieved. Where local conditions made it desirable or necessary the pattern or the scheme of connections can be varied in different parts to achieve a number of zones each uniform -but providing for different rates of heat dissipation. In some cases for instance the intensity of heating may need to be less in the centre as compared with the margins but uniformity is maintained within these zones.

It is to be understood that the heating lilm is only connected to the supply temporarily that is to say when the actu-al heating is performed. Purely resistive heating by connecting the pattern directly to the supply is envisaged. The invention is not concerned with methods or apparatus in which the energy is transferred to the pattern inductively or capacitively.

The invention will be further described with reference to the accompanying diagrammatic drawings in which:

FIGURE l is a plan view of one form of heating lm used in the invention with some parts broken away,

FIGURE 2 is a plan view of an alternative form,

FIGURE 3 is a section which applies equally to FIG- URE l or 2,

FIGURE 4 illustrates the application of the invention lto the drying of paint,

FIGURE 5 illustrates the application of the invention to the curing of plastics, i

FIGURES 6 and 7 illustrate developments of the conductive pattern of the lms of FIGURES 1 and 2 respectively,

FIGURE 8 is a detail section illustrating a further development, i

FIGURE 9 illustrates the application of the invention to an absorber type refrigerator,

FIGURE l0 illustrates the application of the invention to a hair curler,

FIGURE ll illustrates the application of the invention to a towel,

lFIGURE l2 illustrates the application of the invention t-o the ironing of garments.

In the for-m shown in FIGURE 1 the conductive pattern is a .thin flexible metallic foil Il having a series of slots I2 which leave a plurality of parallel meandering paths extending yfrom terminals zones I3 constituted by the areas along the sides of the pattern. I-f the width of the arms of the pattern is kept constant each meandering path will present the same overall resistance and the same resistance per unit length and if a Voltage is maintained from one terminal Zone I3 to the other between the terminal Zones the same current will flow in each path whatever the length of the lm. There will thus be a Y substantially constant dissipation of heat per unit area over the whole film. The film shown in FIGURE 1 can moreover be cut into narrower pieces since any portion between adjacent lines of slots can serve as the terminal zone. It will be clear that however many meander paths are left between terminal zones the cross sectional area of the terminal zones is greater than that of the arms of the meander pattern.

In FIGURE 2 the pattern is similar to that in FIGURE l but the slots I4;- extend across the foil 15 and the terminal zones lr6 are at the ends. Such a film can be cut into pieces by `transverse, cuts as any portion between successive lines of slots can serve as terminal zones.

In some cases the film need not consist of anything more than the metal pattern, but if necessary or desired there may be an insulating film indicated at 17 on one side of the lfoil l1; or l5 and there may be another indicated at 18 on the `other side. Such insulating film may be of plastic or paper for example.

In general the structure in connection With which the film `is used will be a multi-layer structure of which one llayer is the layer containing the absorbed fluid to be heated, and the structure at the time the current is switched on will be at room temperature. Where the heating operation causes vapours to be driven off and these vapours have to escape on the same side as that to which the heating film is applied the heating film will have small passages through and it will be rnade porous or perforated as indicated at 19 FIGURES l rand 2 to allow for the passage of vapour without harm to it or the structure on which the lm is used. Such small passages may be -useful in other cases. In rnany cases the heating film itself will be of cheap construction so that it can be thrown away after once using but the invention also includes cases in which the film is retained for repeated use whether in connection with the same layer or by transfer from one layer to another. Often the heating film is provided with an adhesive coating for example a self-adhesive pressure sensitive material to permit easy fixing to the structure.

It is desira-ble to ensure that the film is to be used repeatedly but on different layers it can be transferred from one to another without suffering damage because if the pattern is damaged and is still conductive, the distribution of resistance will be affected and there will be a risk of hot spots. If on the other hand it is made of cheap construction so that it can be thrown away it should desirably be rnade so flimsy that it is practically irnpossible to avoid destroying it in removing it after use.

In all cases where the pattern can be contacted by a human operator it is so designed that the necessary heating effect can be obtained by a supply of electric current at a low voltage that is to say at a voltage which does not constitute a risk :to the operator if he comes into accidental contact with the pattern. Such voltages are usually below 50 and often well below 50 say l2 volts, 6 volts or even lower. Such voltages als-o make it permissible in many cases e.g. where the layer to be dried is not conductive, to allow the layer to touch the bare pattern. 'Ihese voltages have the advantage of simplifying the construction of the film and its use, since the insulating problems presented by high voltages do not arise. It also makes it possible .to use the heating films for such purposes as hair driers and curlers, and for other domestic purposes such as the drying of foot-wear, clothing, towels and the like or other textile fabrics.

Other `fields of use are in connection with the drying, the drying and curing, or the drying, curing and heating of paint and `similar materials, and of plastic structures and fabrics.

Thus one major field of `application of the heating film is its incorporation in structures which are painted. The incorporation may be temporary or permanent. It is proposed lfor instance to stick a heating film 21 FIGURE 4 on the underside of sheet metal work indicated at 22 before it is painted as at 23. Since the workpiece is metallic there must be an insulating layer between the actual pattern and the workpiece as at 24, but this may be part of the heating film. No insulating layer is shown on the other side of the film 2,1 but it can be provided if desired. The paint is now dried by supplying current to the heating film instead of by the present methods of stoving the panels or 4applying heat to them by other means. Since metal is a good conductor, the heat from the film 21 reaches the paint 23 without a steepy temperature gradient, or in other words the paint 23 is brought to a temperature but little below that reached by the film 2l and due to the lay out of the conductor in the film 2l the heating is substantially uniform `over the whole surface. This not only gives an improvement in drying or curing conventional paints but `it often permits the substitution of stoving enamels for air drying paints. Another major field of application is the incorporation of the film in plastic structures and lfabrics such as low pressure laminates, articles made with potting resins, plastic castings and larger scale mouldings requiring heat for quick setting. FIGURE 5 shows by way of example an embodiment with the heating film 25 (which here can be a bare foil pattern) between two layers 26V of fibre-glass and uncured resin i.e. within the thickness of a plastic structure. The slow drying and curing process of these plastic fabrics is at present hindering the use tof these in many industries requiring speedy output from the moulds, presses or jigs for their formation. 'Ihus the production of glass-fibre structures by the socalled wet-lay process is restricted for mass production applications by the practical difficulty of and equipment required for accelerating the heat drying and curing of the structure. The incorporation of the heating film in such structures enables them to be dried and cured quickly and the moulds to be liberated more frequently with consequent saving in licor space and equipment. Heating films incorporated in plastics preferably have small passages (i.e. small holes or pores) through which the fiuid resin can flow.

It should be mentioned that the term plastic is used herein to indicate not only materials which when cured are substantially rigid but also materials which are elastomeric, such as natural and synthetic rubber, and polyvinyl chloride.

Where a paint ilayer, a plastic or fabric is part of an article which subsequently needs to he heated during use or where heating provision for repairs is desired, the film used for the initial operation 'of drying, curing, etc. may remain in situ and serve subsequently as a heater in the use or repair of t le article. In that case the conductive pattern will be laid out so that it can conveniently serve both purposes. Thus it may be operated at different voltages in its original and subsequent uses or it may be arranged so that it can be reconnected to give a different heat [output when supp-lied with current at the same voltage.

Examples of articles in which the same heating film may be used in the original production of the article and in its subsequent serviceare radiating panels, ironing boards, door mats, vessels such as tanks, beakers, baths and sinks, and pipes, pots, pans, dishes and draining boards.

Yet another field of use of the heating film is in connection with refrigerators of the absorption type.

As above explained, the heating film used in the invention is iessentially a thin flexible pattern, preferably of metallic foil, and can be made by several of the now well known printed circuit techniques. Improved and preferable methods for making these films have been described in my applications Serial Nos. 747,315 and 747,- 314, tiled July 8, 1958, and particularly Serial No. 783,- 633, filed November 10, 1958. The last application describes a heating pattern which is especially suitable for many applications of the present invention when made with aluminium foil coated on both sides. The use of a very low voltage permits the use of stronger foils, and

over-all, additional plastic film support is sometimes not necessary at all.

As also above explained, the pattern will generally be a meander pattern as shown in FIGURES l and 2 in elementary form and as described in these prior applications. Preferably the pattern is such that numerous meander patterns are provided in parallel each pattern covering only a small area of the same convenient module Le. unit size. Then a standard film can be produced in rolls of great width and length and can be cut to required size and shapes when the quantity of articles to which it is to be applied is not great enough to warrant the production of a pattern specially for the job. r[lh-is enables the invention to be applied readily to repair and maintenance work and enables such materials as stoving enamels to be used for this type of work where their use might be otherwise very inconvenient or quite impracticable because the painted surface is in a location or on an article which it would be inconvenient or impossible to transfer to an oven for stoving. rIlle lilm may be provided -with a self-adhesive coating enabling it to be readily applied and stripped after use and can easily be made cheap enough to be dispensible. However with a suitable adhesive it can be stripped without damage and can be re-used, if necessary after a fresh adhesive has been applied to the film lor workpiece.

Even Where stoving would rbe possible the present invention enables ovens or hot presses to be dispensed with and while the iilm may be dispensible, in other cases it may be left permanently in position on the underside of articles having a thin enough wall to make the heat transfer effective, or even between the surf-ace of the article and the 'layer of paint. In this iield lthe invention may be `applied for example to shaped par-ts of automobile body work, sheet metal :parts of domestic equipment such as Washing machines, and metalwork in buildings such `as pipes, doors and windows and can be used for the original paint drying, while thereafter damage to the paint-work can be repaired even with a stoving enamel. In all these cases, if the film is made with a crimped foil pattern as in my said application Serial No. 783,633 the film can readily be applied to intrinsically curved surfaces.

lf the pattern contains numerous repeats with separate terminal zones brought out or accessible at adequate intervals heating can readily be confined to a particular area in the case of local rep-airs or other cases where this is desirable. Thus in FIGURE 6 which shows only the conductive pattern on the lines of FIGURE l1, slots 27 subdivide the marginal terminal zone into separate zones lfor each double meander path While in FIGURE 3 which shows only the conductive pattern on `the lines of FIGURE 2, slots 28 sub-divide the transverse terminal zone into separate zones for each double meander path. It usually suffices however to provide such slots at less frequent interval-s.

As applied to plastic structures and fabrics the film can be applied to the surface or within the thickness lof the material and in the llatter form it is preferably perforated. It is particulally applicable to laminated products made from fibre-glass by the so-ca-lled wet-lay process.

`In all the above cases the heat is conveyed to the layer to be heated by 'conduction and where consideration of strength appearance or the like do not make it objectionable it is advantageous as in FIGURE 8 to use a heat transmitting layer 3l next to the iilm 29 which contains heat conductive particles 32 su-ch as graphite or aluminium or other metal powder to ensure better heat conduction in the desired direction while a heat insulating layer 33 such as glass or asbestos libre is -applied on the other side of the heating iilm to reduce loss of heat in this direction. Such heat insulating layers may be bound with a resin which will resist a higher temperature than that to which the film rises in operation. If necessary a thin electrical insulating layer 34 which will resist the temperature involved may be used between the heating ilm and the graphite or metal filled layer. Such an insulating ilm will be necessary for instance where the heating film itself is a bare metallic pattern.

The layer 34 may be regarded as such a layer or as part of the heating film. The layer 31 may be used without the layer 33 or the layer 33 without the layer 31 with less effect .than when both are used. By providing the heat conductive layer 31 a heavier loading in watts per unit area can `be used without the development of excessive temperatures or hot spots.

In the case of an absorption refrigerator, since the method of Ithe invention enables the heat to be applied over a very large area, the times of the alternate phases of absorption of the vapour and of regeneration of the solid can be substantially shortened. In this particular application the film will be porous or perforated and is preferably made into fiat bags which enclose a very thin layer of the absorbent solids and therefore presenting a maximum surface of the solids both to the vapour coming from the evaporator and to the heat carried at a high rate to the solid from the large surface area of the iilxn.

A suitable arrangement is shown in FIGURE 9. Between a pipe line V35 leading from the evaporator and a pipe line y36 leading to the condenser, the pipe is formed into two branches 37, 38, interlinked change-over valves being provided at 39, 41 so that at any instant one branch is connected to the evaporator and the other to the condenser. In the position shown the branch 37 is connected Ito the evaporator and the branch 38 to the condenser. In each -branch is carried a heating lm bag 42, 43 confining the absorbent powder in a thin layer. The film Iis porous but the pores must be of such size that they do not become choked by the powder. In the position shown the rlilm 43 is being heated so driving off the absorbed vapour tothe condenser while the iilm 42 is not supplied with current and the solids are absorbing the vapours corning from the evaporator. The supply of electricity to the two ilrns is controlled by switches interlinked with the valves 39, 41. The two-branch arrangement and the cycle of operations provides for a const-ant flow from the evaporator to the condenser, but by using 4, 6 or 8 branches instead of only two and 'arranging for the periods to overlap, the vapour flow can be made still smoother.

The branches are preferably very shallow metal bodies with very large surfaces which may be or water-cooled. Cooling lins are diagrammatically indicated at 44. It will be understood .that the ligure shows the narrow depth and the length of the branches and that they also extend to a considerable width in the plane perpendicular to ithe drawing. rThe bags constituted by the film 4Z, 43 are of liat form 'fixed in the central plane of the branches Iand extend practically over the Whole length and width of the branches. The heating film `temperature need not rise much above the [temperature required for regeneration and the small rnass of the iilm enables it to cool and heat rapidly so that changes from one phase to another both yon the iilm and in the powder can take place very quickly. This property together with the large surface area of the lilm enables absorption refrigerators of larger capacity to be built than at present possible.

The present invention can also be applied to the drying of human hair, and wearing apparel ofv any kind from hat to shoe. Indirectly it is also applicable to the hair and skin by -use with combs, hair and other brushes, towels, bandages and plasters. Such devices as hair curlers, inserts or covers for all sorts of clothing includving tie-stretchers, shoe-trees and shoe-cases, may be provided with the heating iilm. At present such devices are mainly -used as purely mechanical aids to tix, stretch or hold the hair or clothing in position or bring it into a desired shape while it is air drying. Towels are only preheated in hot boxes, on towel rails or the like.

For these purposes a porous or perforated heating film with ample openings is used which is connected to a source of very low voltage (battery or secondary of a transformer) to be incorporated 1n or attached to the above devices either permanently or `as a `dispensaible itern (to save cleaning or the expense of making them robust). A pair of insulated wires '(round or flat) connects the film with the supply which is preferably of the order of 2 volts to 12 volts and may be adjustable. In many cases it is belo-w 2 volts. One or both ends of these wires have plug-socket terminations.

The perforations in the film are preferably made after the foil pattern is laminated between two insulating films so that the holes are not clogged in the heating film production process. In many cases the perforations are not only lin the gaps between the metallic areas of the film, but in the metallic areas as well, and often perforated foil is used in producing the heating films.

The film and the metallic pattern has to cover substantially the lwhole surface to -which it is applied or under which it is xed, and follow its outline. In hair curlers as shown for example in FIGURE a plastic material reinforcing and supporting the heating lrn is made thick enough to form the ovoid body 45 round which the hair is wound and fixed, the connecting cable being indicated at 46, a plug connection at 47 and the supply here being exemplified by the secondary of a transformer 48. It will be understood that both the film and the plastic reinforcing material have square or similarly shaped openings @giving them the same meshed lforrn and that the reinforced film itself is rolled up and shaped to form the ovoid body `45. The same scheme is equally applicable to other well known forms of hair curler, e.g. cylindrical or flat. A hair dressing set may consist of say 21/2 dozen hair curlers, each to be plugged into a soclcet having 30 pairs of terminals and a timed common voltage regulation or individual thermal cut-outs, followed by a timed switch-off. The timing can be so programmed that the hairdresser can undo the first curler just after he has put in the last. The timed switch-off would proceed however irrespective of the hairdresser keeping to schedule. Thermostats or heat sensing elements in the curlers are alternative safety provisions.

The provision of a heating film in a comb can be made by placing the metallic pattern in the die in which the comb is cast or moulded by injection or compression. In brushes the heating lm is usually laid in the back of the brush.

Towels and bathrobes `are preferably provided with an expanded, crimped andanodized foil pattern 49, FIG. 1l, without overall film support, fixed to the towel by clips, plastic tape or fabric. About half the size of the towel 51 is usually sufcient for the pattern so that it is enclosed by the towel folded over it. If the foil has been coated with a tough plastic l-m the pattern can stand quite a number of rubs before it is discarded.

Heatable porous materials adapted for application to the human body eg. bandages and adhesive plasters made from porous or perforated plastic film are readily provided with a crimped foil pattern by sticking a selfadhesive perforated heating film on their back, or the heating film may be embedded within the porous material.

Perforated films carrying a metal pattern can be produced as self-adhesive tapes or as shaped labels and fixed to the surface of the covers and Walls of the various drying devices for clothes and shoes. For incorporation into such devices as well as for ironing purposes these films are made without adhesive backing and fixed underneath the surface of the device. For ironing the film 52, FlG. 12, serves as a heatable blanket requiring only pressure by a permeable or absorbent :fabric y53, pad or net to iron the garment 54 sandwiched between two films S2, 55 compressed under the one film 52. The pressure can be exerted by stretching the fabric 53 over a curved support 56, the film preventing creases in the garment 54 to be ironed by not transmitting the shear forces of the stretched fabric. For such purposes it has a slippery surface provided by a backing of a strong perforated metal foil or thin plastic sheet.

-In some cases as well as for drying the film may also serve for heating clothing while it is being worn.

It will be clear from the various examples in the above description that the fixing of the heating lm to the durable structure is effected in principle `in one of two different :ways Either the film is applied e.g. stretched or pressed on or incorporated in the structure, or the structure is pressed on to the film by applying force to the structure eg. stretching or pressing it.

I claim:

l. Structure for drying comprising metal resistance foil, an insulating layer coextensive with said foil, said foil =being connected across a voltage supply at opposite ends through terminal zones, said insulating layer bein-g perforated with a plurality of rows of bore holes, said foil having parallel rows of perforate slots with the slots of one row in staggered relation to the slots of the next succeeding row to form meandering paths for current, and the bore holes of the perforate insulating layer covering the entire area of the insulating layer so that moisture from the surfaces to be dried being transmitted through the holes and slots away from the drying surface.

2. Structure as set forth in claim 1, in which the value of the resistance is such that the voltage drop is not dangerous to human contact.

3. Structure as set forth in claim 1, in which the crosssectional area of said terminal zones is not less than the aggregate cross-sectional area of a pattern corresponding to the total resistance.

4. Structure as set forth in claim 1, in which the insulating layer has particles of material of good heat conductivity distributed through it.

5. Structure as set forth in claim 1, in which a layer of heat insulation is provided on the face of the foil away from the drying surface.

`6. Structure as set forth in claim 1, in which the insulating layer has particles of material of good conductivity distributed through it, and in which a layer of heat insulation is provided on the surface of the foil away from the drying surface.

7. Structure as set forth in claim 1, in which the structure is a porous material adapted for application to the human tbody.

8. Structure as set forth in claim 1, in which a coating is provided of adhesive fixing said foil to said layer.

9. Structure as set forth in claim 1, in which said structure is of plastic and is incorporated within the thickness thereof.

References Cited in the tile of this patent UNITED STATES PATENTS 1,369,323 Sass Nov. 30, 1920 1,624,029 Whitcomb Apr. 12, .1927 1,967,609 Corcoran July 24, 1934 2,627,012 Kinsella Ian. 27, 1953 2,745,942 Cohen May 15, 1956 2,757,273 Taylor July 31, 1956 2,781,439 Lane Feb. 12, 1957 2,787,694 Farries Apr. 2, 1957 2,884,509 Heath Apr. 28, 1959 2,945,115 Weitzel July 12, 1960 FOREIGN PATENTS 719,318 Great Britain Dec. 1, 1954 765,709 Great Britain Jan. 9, 1959 

1. STRUCTURE FOR DRYING COMPRISING METAL RESISTANCE FOIL, AN INSULATING LAYER COEXTENSIVE WITH SAID FOIL, SAID FOIL BEING CONNECTED ACROSS A VOLTAGE SUPPLY AT OPPOSITE ENDS THROUGH TERMINAL ZONES, SAID INSULATING LAYER BEING PERFORATED WITH A PLURALITY OF ROWS OF BORE HOLES, SAID FOIL HAVING PARALLEL ROWS OF PERFORATE SLOTS WITH THE SLOTS OF ONE ROW IN STAGGERED RELATION TO THE SLOTS OF THE NEXT 